theodolite: surveying instrument for measuring horizontal angles, and perhaps vertical angles, see article on the theodolite.
theodolite and sundial: an instrument combining the functions of a sundial and a theodolite.
throne: part of an astrolabe, connecting the instrument to the suspension shackle, see article on the astrolabe.
triangulation: surveying technique involving the measurement of a baseline, the location of other stations by taking angles from either end, and perhaps the extension of the survey through the addition of further triangles.
triangulation instrument: instrument with three jointed arms with scales for surveying or range-finding, see article on the triangulation instrument.